Ferric and acid complexes are known in the art and are useful as photochemical reagents for developing photographic film. Conventional ferric and acid complexes are generally prepared by reacting a ferric salt and an aminocarboxylic acid chelant in water. The aminocarboxylic acid chelant has four carboxylic acid functionalities. Some of the carboxylic acid functionalities are deprotonated with ammonium hydroxide by removing a proton. A hydroxide ion of the ammonium hydroxide and one of the carboxylic acid functionalities react to produce a water molecule. In addition, the ferric salts react with the carboxylic acid functionalities to result in production of iron ions having a positive valence and removal of a proton from more of the carboxylic acid functionalities to produce water molecules. As a result, many of the carboxylic acid functionalities of the chelant are converted into carboxylate ions having a negative valence. Coordinate bonds are formed between one of the iron ions and three of the carboxylate ions of the chelant to produce the ferric and acid complex. After reacting with the carboxylic acid functionalities, the ferric salts provide iron ions having a positive valence.
One of the problems with production of the conventional complexes is that the ferric salts are expensive compared to other sources of iron. As a result, iron oxides have been used as a source of iron ions. To make the iron ions available to react with the carboxylate ions of the aminocarboxylic acid chelant, the iron source must be dissolved in water along with the aminocarboxylic acid chelant. However, iron oxides are relatively insoluble in water and require longer mix times and higher temperatures to react, as compared to the ferric salts, which more readily mix into the water due to their higher solubility. The longer mix times and higher temperatures result in an accelerated rate of degradation of the aminocarboxylic acid chelants. Degradation of the aminocarboxylic acid chelants is undesirable and impacts formation of the ferric and acid complex. Also, by-products formed from the degradation can negatively influence the desired performance of the ferric and acid complex in, for example, the photochemical application. Ferrous salts, which provide Fe2+ ions as opposed to Fe3+ ions, have been used to catalyze the reaction between the iron oxide and the aminocarboxylic acid chelants.
There remains an opportunity to further improve processing parameters for the ferric and acid complexes by using reaction promoters other than or in addition to the ferrous salts to minimize degradation of the chelant during processing.